Welding devices and methods for welding thin thermoplastic resin tubes such as polyethylene tubes have hitherto been put to practical use. Catheters, in particular, are small in diameter as well as in thickness, and a catheter welding method has been known in which catheters with different diameters are fitted one upon the other and the overlaps of the catheters are covered with a heat-shrinkable tube and welded together by making use of the contractive force of the heat-shrinkable tube (see Patent Document 1, by way of example).
However, the method using a heat-shrinkable tube has the following drawbacks: (1) it is difficult to cause the heat-shrinkable tube to shrink uniformly; (2) it is difficult to automate the work of setting the heat-shrinkable tube before welding and the work of removing the heat-shrinkable tube after welding; and (3) the heat-shrinkable tube cannot be reused. There has also been proposed a thermal welding method for joining thermoplastic resin tubes, wherein a pressurizing tube of, for example, silicone rubber is used in place of a heat-shrinkable tube, and thermoplastic resin tubes are thermally welded together with the pressurizing tube stretched in an axial direction thereof such that a welding part to be welded is compressed over its entire circumference (see Patent Document 2, by way of example).
Referring to FIGS. 33A to 33C, the conventional method of welding thermoplastic resin tubes, disclosed in Patent Document 2, will be explained. In this welding method, first, as shown in FIG. 33A, a small-diameter thermoplastic resin tube 52a and then a large-diameter thermoplastic resin tube 52b are fitted on a core (mandrel) 51, which is a supporting rod of metal such as stainless steel, such that distal end portions of the thermoplastic resin tubes are coaxially lapped one over the other, forming overlaps 53. Then, as shown in FIG. 33B, the overlaps 53, which are a welding part to be welded, are inserted into a pressurizing tube 54 held at both ends by left and right chucks 55 and 56 and having good releasability, such as a silicone tube. Subsequently, as shown in FIG. 33C, the right chuck 56 is moved to the right as indicated by arrows in the figure, to stretch the pressurizing tube 54 in an axial direction thereof. When axially stretched, the pressurizing tube 54 narrows in radial directions, that is, in directions perpendicular to the axis of the core 51, and the inner wall of the pressurizing tube 54 compresses the overlaps 53 over their entire circumference. With the overlaps 53 thus compressed, heat is generated by a heat source 57 disposed around the pressurizing tube 54 so that the overlaps 53 may be melted and thermally welded together by external heat. After the thermal welding, the right chuck 56 is moved back to the left as viewed in the figure to restore the original shape of the pressurizing tube 54, and then the thermoplastic resin tubes welded together are taken out.
This conventional method is advantageous in that: (1) the overlaps of the thermoplastic resin tubes can be compressed uniformly; (2) the compressive force of the pressurizing tube can be varied by moving one of the chucks, and the thermoplastic resin tubes can be easily set or removed; and (3) the pressurizing tube can be repeatedly used.
However, the conventional method is still associated with some problems to be solved as the welding method or device for thermoplastic resin tubes, including the disadvantage that the conventional method is dedicated to welding of overlapped thermoplastic resin tubes having different diameters.
Let us suppose the case where, as shown in FIG. 34A, thermoplastic resin tubes 62a and 62b fitted on the core 51 as a supporting member and having the same diameter are subjected to butt welding, by way of example. As shown in FIG. 34B, even if the welding part, that is, the abutting part (abutting faces and their vicinities) of the thermoplastic resin tubes 62a and 62b, is located in the middle of the pressurizing tube 54 (in the figure, the position shifted by L/2 from the left end), the pressurizing tube 54 stretches by ΔL, as shown in FIG. 34C, when the right chuck 56 is moved to the right as indicated by arrows in the figure. Consequently, the midpoint of the pressurizing tube 54 also shifts to the right by ΔL/2. The abutting faces of the thermoplastic resin tubes 62a and 62b tend to remain in the original position because of the force of friction against the core 51, but the inner wall of the pressurizing tube 54 moves in such a direction as to separate the abutting faces from each other. If the abutting faces of the thermoplastic resin tubes 62a and 62b are separated from each other, a space 63 is formed. Such space 63 gives rise to a problem that pinholes are liable to be formed in the welded part.
The above conventional method is also associated with other problems as stated below. It is difficult to compress the outer peripheries of the thermoplastic resin tubes with a desired pressure. Since the pressurizing tube 54 is directly heated by the heat source 57, it is necessary to wait for the temperature of the pressurizing tube 54 to become sufficiently low before the removal or setting of the thermoplastic resin tubes. The conventional method uses external heating and thus is not suited for the connection of tubes of such a combination that the melting point of the outside tube (outer tube) is lower than that of the inner tube (see Patent Document 3, by way of example).
Also, the conventional method is used exclusively for welding and joining thermoplastic resin articles that are already formed into a tube, and does not take into account the case of performing butt welding on a thermoplastic resin sheet wound around the core to obtain a thermoplastic resin tube, the case of performing butt welding on flat thermoplastic resin articles to obtain a flat thermoplastic resin plate, or the case of welding flat thermoplastic resin plates lapped one over the other, for example.